Introduction
Design patterns are solutions to recurring problems guidelines on how to tackle certain problems. They are not classes, packages or libraries that you can plug into your application and wait for the magic to happen. These are, rather, guidelines on how to tackle certain problems in certain situations.
Design patterns solutions to recurring problems guidelines on how to tackle certain problems
Wikipedia describes them as
In software engineering, a software design pattern is a general reusable solution to a commonly occurring problem within a given context in software design. It is not a finished design that can be transformed directly into source or machine code. It is a description or template for how to solve a problem that can be used in many different situations.
Types of Design Patterns
Creational Design Patterns
Creational patterns are focused towards how to instantiate an object or group of related objects.
Wikipedia says
In software engineering, creational design patterns are design patterns that deal with object creation mechanisms, trying to create objects in a manner suitable to the situation. The basic form of object creation could result in design problems or added complexity to the design. Creational design patterns solve this problem by somehow controlling this object creation.
Example:
First of all we have a door interface and the implementation
/*
Door
getWidth()
getHeight()
*/
class WoodenDoor {
constructor(width, height){
this.width = width
this.height = height
}
getWidth(){
return this.width
}
getHeight(){
return this.height
}
}
Then we have our door factory that makes the door and returns it
const DoorFactory = {
makeDoor : (width, height) => new WoodenDoor(width, height)
}
And then it can be used as
const door = DoorFactory.makeDoor(100, 200)
console.log('Width:', door.getWidth())
console.log('Height:', door.getHeight())
When to Use?
When creating an object is not just a few assignments and involves some logic, it makes sense to put it in a dedicated factory instead of repeating the same code everywhere.
Factory Method Design Pattern
Real world example
Consider the case of a hiring manager. It is impossible for one person to interview for each of the positions. Based on the job opening, she has to decide and delegate the interview steps to different people.
In plain words
It provides a way to delegate the instantiation logic to child classes.
Wikipedia says
In class-based programming, the factory method pattern is a creational pattern that uses factory methods to deal with the problem of creating objects without having to specify the exact class of the object that will be created. This is done by creating objects by calling a factory method—either specified in an interface and implemented by child classes, or implemented in a base class and optionally overridden by derived classes—rather than by calling a constructor.
Example:
Taking our hiring manager example above. First of all we have an interviewer interface and some implementations for it
/*
Interviewer interface
askQuestions()
*/
class Developer {
askQuestions() {
console.log('Asking about design patterns!')
}
}
class CommunityExecutive {
askQuestions() {
console.log('Asking about community building')
}
}
Now let us create our HiringManager
class HiringManager {
takeInterview() {
const interviewer = this.makeInterviewer()
interviewer.askQuestions()
}
}
Now any child can extend it and provide the required interviewer
class DevelopmentManager extends HiringManager {
makeInterviewer() {
return new Developer()
}
}
class MarketingManager extends HiringManager {
makeInterviewer() {
return new CommunityExecutive()
}
}
and then it can be used as
const devManager = new DevelopmentManager() devManager.takeInterview() // Output: Asking about design patterns const marketingManager = new MarketingManager() marketingManager.takeInterview() // Output: Asking about community buildng.
When to use?
Useful when there is some generic processing in a class but the required sub-class is dynamically decided at runtime. Or putting it in other words, when the client doesn’t know what exact sub-class it might need.
Abstract Factory Design Pattern
Real world example
Extending our door example from Simple Factory. Based on your needs you might get a wooden door from a wooden door shop, iron door from an iron shop or a PVC door from the relevant shop. Plus you might need a guy with different kind of specialities to fit the door, for example a carpenter for wooden door, welder for iron door etc. As you can see there is a dependency between the doors now, wooden door needs carpenter, iron door needs a welder etc.
In plain words
A factory of factories a factory that groups the individual but related/dependent factories together without specifying their concrete classes.
Wikipedia says
The abstract factory pattern provides a way to encapsulate a group of individual factories that have a common theme without specifying their concrete classes
Example:
Translating the door example above. First of all we have our Door interface and some implementation for it
/*
Door interface :
getDescription()
*/
class WoodenDoor {
getDescription() {
console.log('I am a wooden door')
}
}
class IronDoor {
getDescription() {
console.log('I am an iron door')
}
}
Then we have some fitting experts for each door type
/*
DoorFittingExpert interface :
getDescription()
*/
class Welder {
getDescription() {
console.log('I can only fit iron doors')
}
}
class Carpenter {
getDescription() {
console.log('I can only fit wooden doors')
}
}
Now we have our abstract factory that would let us make family of related objects i.e. wooden door factory would create a wooden door and wooden door fitting expert and iron door factory would create an iron door and iron door fitting expert
/*
DoorFactory interface :
makeDoor()
makeFittingExpert()
*/
// Wooden factory to return carpenter and wooden door
class WoodenDoorFactory {
makeDoor(){
return new WoodenDoor()
}
makeFittingExpert() {
return new Carpenter()
}
}
// Iron door factory to get iron door and the relevant fitting expert
class IronDoorFactory {
makeDoor(){
return new IronDoor()
}
makeFittingExpert() {
return new Welder()
}
}
And then it can be used as
woodenFactory = new WoodenDoorFactory() door = woodenFactory.makeDoor() expert = woodenFactory.makeFittingExpert() door.getDescription() // Output: I am a wooden door expert.getDescription() // Output: I can only fit wooden doors // Same for Iron Factory ironFactory = new IronDoorFactory() door = ironFactory.makeDoor() expert = ironFactory.makeFittingExpert() door.getDescription() // Output: I am an iron door expert.getDescription() // Output: I can only fit iron doors
As you can see the wooden door factory has encapsulated the carpenter and the wooden door also iron door factory has encapsulated the iron door and welder. And thus it had helped us make sure that for each of the created door, we do not get a wrong fitting expert.
When to use?
When there are interrelated dependencies with not-that-simple creation logic involved
Builder Design Pattern
Real world example
Imagine you are at Hardee’s and you order a specific deal, lets say, “Big Hardee” and they hand it over to you without any questions this is the example of simple factory. But there are cases when the creation logic might involve more steps. For example you want a customized Subway deal, you have several options in how your burger is made e.g what bread do you want? what types of sauces would you like? What cheese would you want? etc. In such cases builder pattern comes to the rescue.
In plain words
Allows you to create different flavors of an object while avoiding constructor pollution. Useful when there could be several flavors of an object. Or when there are a lot of steps involved in creation of an object.
Wikipedia says
The builder pattern is an object creation software design pattern with the intentions of finding a solution to the telescoping constructor anti-pattern.
Having said that let me add a bit about what telescoping constructor anti-pattern is. At one point or the other we have all seen a constructor like below:
constructor(size, cheese = true, pepperoni = true, tomato = false, lettuce = true) {
// ...
}
As you can see the number of constructor parameters can quickly get out of hand and it might become difficult to understand the arrangement of parameters. Plus this parameter list could keep on growing if you would want to add more options in future. This is called telescoping constructor anti-pattern.
Example:
The sane alternative is to use the builder pattern. First of all we have our burger that we want to make
class Burger {
constructor(builder) {
this.size = builder.size
this.cheeze = builder.cheeze || false
this.pepperoni = builder.pepperoni || false
this.lettuce = builder.lettuce || false
this.tomato = builder.tomato || false
}
}
And then we have the builder
class BurgerBuilder {
constructor(size) {
this.size = size
}
addPepperoni() {
this.pepperoni = true
return this
}
addLettuce() {
this.lettuce = true
return this
}
addCheeze() {
this.cheeze = true
return this
}
addTomato() {
this.tomato = true
return this
}
build() {
return new Burger(this)
}
}
And then it can be used as:
const burger = (new BurgerBuilder(14))
.addPepperoni()
.addLettuce()
.addTomato()
.build()
Javascript specific tip : When you find that the number of arguments to a function or method are too many (normally any more than 2 arguments is considered too much), use a single object argument instead of multiple arguments. This serves two purposes :
- It makes your code look less cluttered, since there is only one argument.
- You don’t have to worry about the order of arguments since arguments are now passed as named properties of the object.
For example :
const burger = new Burger({
size : 14,
pepperoni : true,
cheeze : false,
lettuce : true,
tomato : true
})
instead of :
const burger = new Burger(14, true, false, true, true)
When to use?
When there could be several flavors of an object and to avoid the constructor telescoping. The key difference from the factory pattern is that factory pattern is to be used when the creation is a one step process while builder pattern is to be used when the creation is a multi step process.
Singleton Design Pattern
Real world example
There can only be one president of a country at a time. The same president has to be brought to action, whenever duty calls. President here is singleton.
In plain words
Ensures that only one object of a particular class is ever created.
Wikipedia says
In software engineering, the singleton pattern is a software design pattern that restricts the instantiation of a class to one object. This is useful when exactly one object is needed to coordinate actions across the system.
Singleton pattern is actually considered an anti-pattern and overuse of it should be avoided. It is not necessarily bad and could have some valid use-cases but should be used with caution because it introduces a global state in your application and change to it in one place could affect in the other areas and it could become pretty difficult to debug. The other bad thing about them is it makes your code tightly coupled plus it mocking the singleton could be difficult.
Example:
In javascript, singletons can be implemented using the module pattern. Private variables and functions are hidden in a function closure, and public methods are selectively exposed.
const president = (function(){
const presidentsPrivateInformation = 'Super private'
const name = 'Turd Sandwich'
const getName = () => name
return {
getName
}
}())
Here, presidentsPrivateInformation and name are kept private. However, name can be accessed with the exposed president.getName method.
president.getName() // Outputs 'Turd Sandwich' president.name // Outputs undefined president.presidentsPrivateInformation // Outputs undefined
Structural Design Patterns
In plain words
Structural patterns are mostly concerned with object composition or in other words how the entities can use each other. Or yet another explanation would be, they help in answering “How to build a software component?”
Wikipedia says
In software engineering, structural design patterns are design patterns that ease the design by identifying a simple way to realize relationships between entities.
- Adapter Design Pattern
- Bridge Design Pattern
- Composite Design Pattern
- Decorator Design Pattern
- Facade Design Pattern
- Flyweight Design Pattern
- Proxy Design Pattern
Adapter Design Pattern
Real world example
Consider that you have some pictures in your memory card and you need to transfer them to your computer. In order to transfer them you need some kind of adapter that is compatible with your computer ports so that you can attach memory card to your computer. In this case card reader is an adapter. Another example would be the famous power adapter a three legged plug can’t be connected to a two pronged outlet, it needs to use a power adapter that makes it compatible with the two pronged outlet. Yet another example would be a translator translating words spoken by one person to another
In plain words
Adapter pattern lets you wrap an otherwise incompatible object in an adapter to make it compatible with another class.
Wikipedia says
In software engineering, the adapter pattern is a software design pattern that allows the interface of an existing class to be used as another interface. It is often used to make existing classes work with others without modifying their source code.
Example:
Consider a game where there is a hunter and he hunts lions.
First we have an interface Lion that all types of lions have to implement
/*
Lion interface :
roar()
*/
class AfricanLion {
roar() {}
}
class AsianLion {
roar() {}
}
And hunter expects any implementation of Lion interface to hunt.
class Hunter {
hunt(lion) {
// ... some code before
lion.roar()
//... some code after
}
}
Now let’s say we have to add a WildDog in our game so that hunter can hunt that also. But we can’t do that directly because dog has a different interface. To make it compatible for our hunter, we will have to create an adapter that is compatible
// This needs to be added to the game
class WildDog {
bark() {
}
}
// Adapter around wild dog to make it compatible with our game
class WildDogAdapter {
constructor(dog) {
this.dog = dog;
}
roar() {
this.dog.bark();
}
}
And now the WildDog can be used in our game using WildDogAdapter.
wildDog = new WildDog() wildDogAdapter = new WildDogAdapter(wildDog) hunter = new Hunter() hunter.hunt(wildDogAdapter)
Bridge Design Pattern
Real world example
Consider you have a website with different pages and you are supposed to allow the user to change the theme. What would you do? Create multiple copies of each of the pages for each of the themes or would you just create separate theme and load them based on the user’s preferences? Bridge pattern allows you to do the second i.e.
In Plain Words
Bridge pattern is about preferring composition over inheritance. Implementation details are pushed from a hierarchy to another object with a separate hierarchy.
Wikipedia says
The bridge pattern is a design pattern used in software engineering that is meant to “decouple an abstraction from its implementation so that the two can vary independently”
Example:
Translating our WebPage example from above. Here we have the WebPage hierarchy
/*
Webpage interface :
constructor(theme)
getContent()
*/
class About{
constructor(theme) {
this.theme = theme
}
getContent() {
return "About page in " + this.theme.getColor()
}
}
class Careers{
constructor(theme) {
this.theme = theme
}
getContent() {
return "Careers page in " + this.theme.getColor()
}
}
And the separate theme hierarchy
/*
Theme interface :
getColor()
*/
class DarkTheme{
getColor() {
return 'Dark Black'
}
}
class LightTheme{
getColor() {
return 'Off white'
}
}
class AquaTheme{
getColor() {
return 'Light blue'
}
}
And both the hierarchies
const darkTheme = new DarkTheme() const about = new About(darkTheme) const careers = new Careers(darkTheme) console.log(about.getContent() )// "About page in Dark Black" console.log(careers.getContent() )// "Careers page in Dark Black"
Composite Design Pattern
Real world example
Every organization is composed of employees. Each of the employees has same features i.e. has a salary, has some responsibilities, may or may not report to someone, may or may not have some subordinates etc.
In plain words
Composite pattern lets clients to treat the individual objects in a uniform manner.
Wikipedia says
In software engineering, the composite pattern is a partitioning design pattern. The composite pattern describes that a group of objects is to be treated in the same way as a single instance of an object. The intent of a composite is to “compose” objects into tree structures to represent part-whole hierarchies. Implementing the composite pattern lets clients treat individual objects and compositions uniformly.
Example:
Taking our employees example from above. Here we have different employee types
/*
Employee interface :
constructor(name, salary)
getName()
setSalary()
getSalary()
getRoles()
*/
class Developer {
constructor(name, salary) {
this.name = name
this.salary = salary
}
getName() {
return this.name
}
setSalary(salary) {
this.salary = salary
}
getSalary() {
return this.salary
}
getRoles() {
return this.roles
}
develop() {
/* */
}
}
class Designer {
constructor(name, salary) {
this.name = name
this.salary = salary
}
getName() {
return this.name
}
setSalary(salary) {
this.salary = salary
}
getSalary() {
return this.salary
}
getRoles() {
return this.roles
}
design() {
/* */
}
}
Then we have an organization which consists of several different types of employees
class Organization {
constructor(){
this.employees = []
}
addEmployee(employee) {
this.employees.push(employee)
}
getNetSalaries() {
let netSalary = 0
this.employees.forEach(employee => {
netSalary += employee.getSalary()
})
return netSalary
}
}
And then it can be used as
// Prepare the employees
const john = new Developer('John Doe', 12000)
const jane = new Designer('Jane', 10000)
// Add them to organization
const organization = new Organization()
organization.addEmployee(john)
organization.addEmployee(jane)
console.log("Net salaries: " , organization.getNetSalaries()) // Net Salaries: 22000
Decorator Design Pattern
Real world example
Imagine you run a car service shop offering multiple services. Now how do you calculate the bill to be charged? You pick one service and dynamically keep adding to it the prices for the provided services till you get the final cost. Here each type of service is a decorator.
In plain words
Decorator pattern lets you dynamically change the behavior of an object at run time by wrapping them in an object of a decorator class.
Wikipedia says
In object-oriented programming, the decorator pattern is a design pattern that allows behavior to be added to an individual object, either statically or dynamically, without affecting the behavior of other objects from the same class. The decorator pattern is often useful for adhering to the Single Responsibility Principle, as it allows functionality to be divided between classes with unique areas of concern.
Example:
Lets take coffee for example. First of all we have a simple coffee implementing the coffee interface
/*
Coffee interface:
getCost()
getDescription()
*/
class SimpleCoffee{
getCost() {
return 10
}
getDescription() {
return 'Simple coffee'
}
}
We want to make the code extensible to allow options to modify it if required. Lets make some add-ons (decorators)
class MilkCoffee {
constructor(coffee) {
this.coffee = coffee
}
getCost() {
return this.coffee.getCost() + 2
}
getDescription() {
return this.coffee.getDescription() + ', milk'
}
}
class WhipCoffee {
constructor(coffee) {
this.coffee = coffee
}
getCost() {
return this.coffee.getCost() + 5
}
getDescription() {
return this.coffee.getDescription() + ', whip'
}
}
class VanillaCoffee {
constructor(coffee) {
this.coffee = coffee
}
getCost() {
return this.coffee.getCost() + 3
}
getDescription() {
return this.coffee.getDescription() + ', vanilla'
}
}
Lets make a coffee now
let someCoffee someCoffee = new SimpleCoffee() console.log(someCoffee.getCost())// 10 console.log(someCoffee.getDescription())// Simple Coffee someCoffee = new MilkCoffee(someCoffee) console.log(someCoffee.getCost())// 12 console.log(someCoffee.getDescription())// Simple Coffee, milk someCoffee = new WhipCoffee(someCoffee) console.log(someCoffee.getCost())// 17 console.log(someCoffee.getDescription())// Simple Coffee, milk, whip someCoffee = new VanillaCoffee(someCoffee) console.log(someCoffee.getCost())// 20 console.log(someCoffee.getDescription())// Simple Coffee, milk, whip, vanilla
Facade Design Pattern
Real world example
How do you turn on the computer? “Hit the power button” you say! That is what you believe because you are using a simple interface that computer provides on the outside, internally it has to do a lot of stuff to make it happen. This simple interface to the complex subsystem is a facade.
In plain words
Facade pattern provides a simplified interface to a complex subsystem.
Wikipedia says
A facade is an object that provides a simplified interface to a larger body of code, such as a class library.
Example: Taking our computer example from above. Here we have the computer class
class Computer {
getElectricShock() {
console.log('Ouch!')
}
makeSound() {
console.log('Beep beep!')
}
showLoadingScreen() {
console.log('Loading..')
}
bam() {
console.log('Ready to be used!')
}
closeEverything() {
console.log('Bup bup bup buzzzz!')
}
sooth() {
console.log('Zzzzz')
}
pullCurrent() {
console.log('Haaah!')
}
}
Here we have the facade
class ComputerFacade
{
constructor(computer) {
this.computer = computer
}
turnOn() {
this.computer.getElectricShock()
this.computer.makeSound()
this.computer.showLoadingScreen()
this.computer.bam()
}
turnOff() {
this.computer.closeEverything()
this.computer.pullCurrent()
this.computer.sooth()
}
}
Now to use the facade
const computer = new ComputerFacade(new Computer()) computer.turnOn() // Ouch! Beep beep! Loading.. Ready to be used! computer.turnOff() // Bup bup buzzz! Haah! Zzzzz
Flyweight Design Pattern
Real world example
Did you ever have fresh tea from some stall? They often make more than one cup that you demanded and save the rest for any other customer so to save the resources e.g. gas etc. Flyweight pattern is all about that i.e. sharing.
In plain words
It is used to minimize memory usage or computational expenses by sharing as much as possible with similar objects.
Wikipedia says
In computer programming, flyweight is a software design pattern. A flyweight is an object that minimizes memory use by sharing as much data as possible with other similar objects it is a way to use objects in large numbers when a simple repeated representation would use an unacceptable amount of memory.
Example: Translating our tea example from above. First of all we have tea types and tea maker
// Anything that will be cached is flyweight.
// Types of tea here will be flyweights.
class KarakTea {
}
// Acts as a factory and saves the tea
class TeaMaker {
constructor(){
this.availableTea = {}
}
make(preference) {
this.availableTea[preference] = this.availableTea[preference] || (new KarakTea())
return this.availableTea[preference]
}
}
Then we have the TeaShop which takes orders and serves them
class TeaShop {
constructor(teaMaker) {
this.teaMaker = teaMaker
this.orders = []
}
takeOrder(teaType, table) {
this.orders[table] = this.teaMaker.make(teaType)
}
serve() {
this.orders.forEach((order, index) => {
console.log('Serving tea to table#' + index)
})
}
}
And it can be used as below
const teaMaker = new TeaMaker()
const shop = new TeaShop(teaMaker)
shop.takeOrder('less sugar', 1)
shop.takeOrder('more milk', 2)
shop.takeOrder('without sugar', 5)
shop.serve()
// Serving tea to table# 1
// Serving tea to table# 2
// Serving tea to table# 5
Proxy Design Pattern
Real world example
Have you ever used an access card to go through a door? There are multiple options to open that door i.e. it can be opened either using access card or by pressing a button that bypasses the security. The door’s main functionality is to open but there is a proxy added on top of it to add some functionality. Let me better explain it using the code example below.
In plain words
Using the proxy pattern, a class represents the functionality of another class.
Wikipedia says
A proxy, in its most general form, is a class functioning as an interface to something else. A proxy is a wrapper or agent object that is being called by the client to access the real serving object behind the scenes. Use of the proxy can simply be forwarding to the real object, or can provide additional logic. In the proxy extra functionality can be provided, for example caching when operations on the real object are resource intensive, or checking preconditions before operations on the real object are invoked.
Example: Taking our security door example from above. Firstly we have the door interface and an implementation of door
/*
Door interface :
open()
close()
*/
class LabDoor {
open() {
console.log('Opening lab door')
}
close() {
console.log('Closing the lab door')
}
}
Then we have a proxy to secure any doors that we want
class Security {
constructor(door) {
this.door = door
}
open(password) {
if (this.authenticate(password)) {
this.door.open()
} else {
console.log('Big no! It ain\'t possible.')
}
}
authenticate(password) {
return password === 'ecr@t'
}
close() {
this.door.close()
}
}
And here is how it can be used
const door = new Security(new LabDoor())
door.open('invalid') // Big no! It ain't possible.
door.open('ecr@t') // Opening lab door
door.close() // Closing lab door
Behavioral Design Patterns
In plain words
It is concerned with assignment of responsibilities between the objects. What makes them different from structural patterns is they don’t just specify the structure but also outline the patterns for message passing/communication between them. Or in other words, they assist in answering “How to run a behavior in software component?”
Wikipedia says
In software engineering, behavioral design patterns are design patterns that identify common communication patterns between objects and realize these patterns. By doing so, these patterns increase flexibility in carrying out this communication.
- Chain of Responsibility Design Pattern
- Command Design Pattern
- Iterator Design Pattern
- Mediator Design Pattern
- Memento Design Pattern
- Observer Design Pattern
- Visitor Design Pattern
- Strategy Design Pattern
- State Design Pattern
- Template Method Design Pattern
Chain of Responsibility Design Pattern
Real world example
For example, you have three payment methods (A, B and C) setup in your account each having a different amount in it. A has 100 USD, B has 300 USD and C having 1000 USD and the preference for payments is chosen as A then B then C. You try to purchase something that is worth 210 USD. Using Chain of Responsibility, first of all account A will be checked if it can make the purchase, if yes purchase will be made and the chain will be broken. If not, request will move forward to account B checking for amount if yes chain will be broken otherwise the request will keep forwarding till it finds the suitable handler. Here A, B and C are links of the chain and the whole phenomenon is Chain of Responsibility.
In plain words
It helps building a chain of objects. Request enters from one end and keeps going from object to object till it finds the suitable handler.
Wikipedia says
In object-oriented design, the chain-of-responsibility pattern is a design pattern consisting of a source of command objects and a series of processing objects. Each processing object contains logic that defines the types of command objects that it can handle the rest are passed to the next processing object in the chain.
Example:
Translating our account example above. First of all we have a base account having the logic for chaining the accounts together and some accounts
class Account {
setNext(account) {
this.successor = account
}
pay(amountToPay) {
if (this.canPay(amountToPay)) {
console.log(`Paid ${amountToPay} using ${this.name}`)
} else if (this.successor) {
console.log(`Cannot pay using ${this.name}. Proceeding...`)
this.successor.pay(amountToPay)
} else {
console.log('None of the accounts have enough balance')
}
}
canPay(amount) {
return this.balance >= amount
}
}
class Bank extends Account {
constructor(balance) {
super()
this.name = 'bank'
this.balance = balance
}
}
class Paypal extends Account {
constructor(balance) {
super()
this.name = 'Paypal'
this.balance = balance
}
}
class Bitcoin extends Account {
constructor(balance) {
super()
this.name = 'bitcoin'
this.balance = balance
}
}
Now let’s prepare the chain using the links defined above (i.e. Bank, Paypal, Bitcoin)
// Let's prepare a chain like below // bank.paypal.bitcoin // // First priority bank // If bank can't pay then paypal // If paypal can't pay then bit coin const bank = new Bank(100) // Bank with balance 100 const paypal = new Paypal(200) // Paypal with balance 200 const bitcoin = new Bitcoin(300) // Bitcoin with balance 300 bank.setNext(paypal) paypal.setNext(bitcoin) // Let's try to pay using the first priority i.e. bank bank.pay(259) // Output will be // ============== // Cannot pay using bank. Proceeding .. // Cannot pay using paypal. Proceeding ..: // Paid 259 using Bitcoin!
Command Design Pattern
Real world example
A generic example would be you ordering a food at restaurant. You (i.e. Client) ask the waiter (i.e. Invoker) to bring some food (i.e. Command) and waiter simply forwards the request to Chef (i.e. Receiver) who has the knowledge of what and how to cook. Another example would be you (i.e. Client) switching on (i.e. Command) the television (i.e. Receiver) using a remote control (Invoker).
In plain words
Allows you to encapsulate actions in objects. The key idea behind this pattern is to provide the means to decouple client from receiver.
Wikipedia says
In object-oriented programming, the command pattern is a behavioral design pattern in which an object is used to encapsulate all information needed to perform an action or trigger an event at a later time. This information includes the method name, the object that owns the method and values for the method parameters.
Example:
First of all we have the receiver that has the implementation of every action that could be performed
// Receiver
class Bulb {
turnOn() {
console.log('Bulb has been lit')
}
turnOff() {
console.log('Darkness!')
}
}
then we have an interface that each of the commands are going to implement and then we have a set of commands
/*
Command interface :
execute()
undo()
redo()
*/
// Command
class TurnOnCommand {
constructor(bulb) {
this.bulb = bulb
}
execute() {
this.bulb.turnOn()
}
undo() {
this.bulb.turnOff()
}
redo() {
this.execute()
}
}
class TurnOffCommand {
constructor(bulb) {
this.bulb = bulb
}
execute() {
this.bulb.turnOff()
}
undo() {
this.bulb.turnOn()
}
redo() {
this.execute()
}
}
Then we have an Invoker with whom the client will interact to process any commands
// Invoker
class RemoteControl {
submit(command) {
command.execute()
}
}
Finally let’s see how we can use it in our client
const bulb = new Bulb() const turnOn = new TurnOnCommand(bulb) const turnOff = new TurnOffCommand(bulb) const remote = new RemoteControl() remote.submit(turnOn) // Bulb has been lit! remote.submit(turnOff) // Darkness!
Command pattern can also be used to implement a transaction based system. Where you keep maintaining the history of commands as soon as you execute them. If the final command is successfully executed, all good otherwise just iterate through the history and keep executing the undo on all the executed commands.
Iterator Design Pattern
Real world example
An old radio set will be a good example of iterator, where user could start at some channel and then use next or previous buttons to go through the respective channels. Or take an example of MP3 player or a TV set where you could press the next and previous buttons to go through the consecutive channels or in other words they all provide an interface to iterate through the respective channels, songs or radio stations.
In plain words
It presents a way to access the elements of an object without exposing the underlying presentation.
Wikipedia says
In object-oriented programming, the iterator pattern is a design pattern in which an iterator is used to traverse a container and access the container’s elements. The iterator pattern decouples algorithms from containers in some cases, algorithms are necessarily container-specific and thus cannot be decoupled.
Example: Translating our radio stations example from above. First of all we have RadioStation
class RadioStation {
constructor(frequency) {
this.frequency = frequency
}
getFrequency() {
return this.frequency
}
}
Then we have our iterator
class StationList {
constructor(){
this.stations = []
}
addStation(station) {
this.stations.push(station)
}
removeStation(toRemove) {
const toRemoveFrequency = toRemove.getFrequency()
this.stations = this.stations.filter(station => {
return station.getFrequency() !== toRemoveFrequency
})
}
}
And then it can be used as
const stationList = new StationList() stationList.addStation(new RadioStation(89)) stationList.addStation(new RadioStation(101)) stationList.addStation(new RadioStation(102)) stationList.addStation(new RadioStation(103.2)) stationList.stations.forEach(station => console.log(station.getFrequency())) stationList.removeStation(new RadioStation(89)) // Will remove station 89
Mediator Design Pattern
Real world example
A general example would be when you talk to someone on your mobile phone, there is a network provider sitting between you and them and your conversation goes through it instead of being directly sent. In this case network provider is mediator.
In plain words
Mediator pattern adds a third party object (called mediator) to control the interaction between two objects (called colleagues). It helps reduce the coupling between the classes communicating with each other. Because now they don’t need to have the knowledge of each other’s implementation.
Wikipedia says
In software engineering, the mediator pattern defines an object that encapsulates how a set of objects interact. This pattern is considered to be a behavioral pattern due to the way it can alter the program’s running behavior.
Example:
Here is the simplest example of a chat room (i.e. mediator) with users (i.e. colleagues) sending messages to each other.
First of all, we have the mediator i.e. the chat room
// Mediator
class ChatRoom {
showMessage(user, message) {
const time = new Date()
const sender = user.getName()
console.log(time + '[' + sender + ']:' + message)
}
}
Then we have our users i.e. colleagues
class User {
constructor(name, chatMediator) {
this.name = name
this.chatMediator = chatMediator
}
getName() {
return this.name
}
send(message) {
this.chatMediator.showMessage(this, message)
}
}
And the usage
const mediator = new ChatRoom()
const john = new User('John Doe', mediator)
const jane = new User('Jane Doe', mediator)
john.send('Hi there!')
jane.send('Hey!')
// Output will be
// Feb 14, 10:58 [John]: Hi there!
// Feb 14, 10:58 [Jane]: Hey!
Memento Design Pattern
Real world example
Take the example of calculator (i.e. originator), where whenever you perform some calculation the last calculation is saved in memory (i.e. memento) so that you can get back to it and maybe get it restored using some action buttons (i.e. caretaker).
In plain words
Memento pattern is about capturing and storing the current state of an object in a manner that it can be restored later on in a smooth manner.
Wikipedia says
The memento pattern is a software design pattern that provides the ability to restore an object to its previous state (undo via rollback).
Usually useful when you need to provide some sort of undo functionality.
Example:
Lets take an example of text editor which keeps saving the state from time to time and that you can restore if you want.
First of all we have our memento object that will be able to hold the editor state
class EditorMemento {
constructor(content) {
this._content = content
}
getContent() {
return this._content
}
}
Then we have our editor i.e. originator that is going to use memento object
class Editor {
constructor(){
this._content = ''
}
type(words) {
this._content = this._content + ' ' + words
}
getContent() {
return this._content
}
save() {
return new EditorMemento(this._content)
}
restore(memento) {
this._content = memento.getContent()
}
}
And then it can be used as
const editor = new Editor()
// Type some stuff
editor.type('This is the first sentence.')
editor.type('This is second.')
// Save the state to restore to : This is the first sentence. This is second.
const saved = editor.save()
// Type some more
editor.type('And this is third.')
// Output: Content before Saving
console.log(editor.getContent())// This is the first sentence. This is second. And this is third.
// Restoring to last saved state
editor.restore(saved)
console.log(editor.getContent()) // This is the first sentence. This is second.
Observer Design Pattern
(Otherwise known as “pub-sub”)
Real world example
A good example would be the job seekers where they subscribe to some job posting site and they are notified whenever there is a matching job opportunity.
In plain words
Defines a dependency between objects so that whenever an object changes its state, all its dependents are notified.
Wikipedia says
The observer pattern is a software design pattern in which an object, called the subject, maintains a list of its dependents, called observers, and notifies them automatically of any state changes, usually by calling one of their methods.
Example:
Translating our example from above. First of all we have job seekers that need to be notified for a job posting
const JobPost = title => ({
title: title
})
class JobSeeker {
constructor(name) {
this._name = name
}
notify(jobPost) {
console.log(this._name, 'has been notified of a new posting :', jobPost.title)
}
}
Then we have our job postings to which the job seekers will subscribe
class JobBoard {
constructor() {
this._subscribers = []
}
subscribe(jobSeeker) {
this._subscribers.push(jobSeeker)
}
addJob(jobPosting) {
this._subscribers.forEach(subscriber => {
subscriber.notify(jobPosting)
})
}
}
Then it can be used as
// Create subscribers
const jonDoe = new JobSeeker('John Doe')
const janeDoe = new JobSeeker('Jane Doe')
const kaneDoe = new JobSeeker('Kane Doe')
// Create publisher and attach subscribers
const jobBoard = new JobBoard()
jobBoard.subscribe(jonDoe)
jobBoard.subscribe(janeDoe)
// Add a new job and see if subscribers get notified
jobBoard.addJob(JobPost('Software Engineer'))
// Output
// John Doe has been notified of a new posting : Software Engineer
// Jane Doe has been notified of a new posting : Software Engineer
Visitor Design Pattern
Real world example
Consider someone visiting Dubai. They just need a way (i.e. visa) to enter Dubai. After arrival, they can come and visit any place in Dubai on their own without having to ask for permission or to do some leg work in order to visit any place here just let them know of a place and they can visit it. Visitor pattern let’s you do just that, it helps you add places to visit so that they can visit as much as they can without having to do any legwork.
In plain words
Visitor pattern let’s you add further operations to objects without having to modify them.
Wikipedia says
In object-oriented programming and software engineering, the visitor design pattern is a way of separating an algorithm from an object structure on which it operates. A practical result of this separation is the ability to add new operations to existing object structures without modifying those structures. It is one way to follow the open/closed principle.
Example:
Let’s take an example of a zoo simulation where we have several different kinds of animals and we have to make them Sound. Let’s translate this using visitor pattern
We have our implementations for the animals
class Monkey {
shout() {
console.log('Ooh oo aa aa!')
}
accept(operation) {
operation.visitMonkey(this)
}
}
class Lion {
roar() {
console.log('Roaaar!')
}
accept(operation) {
operation.visitLion(this)
}
}
class Dolphin {
speak() {
console.log('Tuut tuttu tuutt!')
}
accept(operation) {
operation.visitDolphin(this)
}
}
Let’s implement our visitor
const speak = {
visitMonkey(monkey){
monkey.shout()
},
visitLion(lion){
lion.roar()
},
visitDolphin(dolphin){
dolphin.speak()
}
}
And then it can be used as
const monkey = new Monkey() const lion = new Lion() const dolphin = new Dolphin() monkey.accept(speak) // Ooh oo aa aa! lion.accept(speak) // Roaaar! dolphin.accept(speak) // Tuut tutt tuutt!
We could have done this simply by having a inheritance hierarchy for the animals but then we would have to modify the animals whenever we would have to add new actions to animals. But now we will not have to change them. For example, let’s say we are asked to add the jump behavior to the animals, we can simply add that by creating a new visitor i.e.
const jump = {
visitMonkey(monkey) {
console.log('Jumped 20 feet high! on to the tree!')
},
visitLion(lion) {
console.log('Jumped 7 feet! Back on the ground!')
},
visitDolphin(dolphin) {
console.log('Walked on water a little and disappeared')
}
}
And for the usage
monkey.accept(speak) // Ooh oo aa aa! monkey.accept(jump) // Jumped 20 feet high! on to the tree! lion.accept(speak) // Roaaar! lion.accept(jump) // Jumped 7 feet! Back on the ground! dolphin.accept(speak) // Tuut tutt tuutt! dolphin.accept(jump) // Walked on water a little and disappeared
Strategy Design Pattern
Real world example
Consider the example of sorting, we implemented bubble sort but the data started to grow and bubble sort started getting very slow. In order to tackle this we implemented Quick sort. But now although the quick sort algorithm was doing better for large datasets, it was very slow for smaller datasets. In order to handle this we implemented a strategy where for small datasets, bubble sort will be used and for larger, quick sort.
In plain words
Strategy pattern allows you to switch the algorithm or strategy based upon the situation.
Wikipedia says
In computer programming, the strategy pattern (also known as the policy pattern) is a behavioural software design pattern that enables an algorithm’s behavior to be selected at runtime.
Example:
Translating our example from above, we can easily implement this strategy in javascript using its feature of first class functions.
const bubbleSort = dataset => {
console.log('Sorting with bubble sort')
// ...
// ...
return dataset
}
const quickSort = dataset => {
console.log('Sorting with quick sort')
// ...
// ...
return dataset
}
And then we have our client that is going to use any strategy
const sorter = dataset => {
if(dataset.length > 5){
return quickSort
} else {
return bubbleSort
}
}
And it can be used as
const longDataSet = [1, 5, 4, 3, 2, 8] const shortDataSet = [1, 5, 4] const sorter1 = sorter(longDataSet) const sorter2 = sorter(shortDataSet) sorter1(longDataSet) // Output : Sorting with quick sort sorter2(shortDataSet) // Output : Sorting with bubble sort
State Design Pattern
Real world example
Imagine you are using some drawing application, you choose the paint brush to draw. Now the brush changes it’s behavior based on the selected color i.e. if you have chosen red color it will draw in red, if blue then it will be in blue etc.
In plain words
It lets you change the behavior of a class when the state changes.
Wikipedia says
The state pattern is a behavioral software design pattern that implements a state machine in an object-oriented way. With the state pattern, a state machine is implemented by implementing each individual state as a derived class of the state pattern interface, and implementing state transitions by invoking methods defined by the pattern’s superclass. The state pattern can be interpreted as a strategy pattern which is able to switch the current strategy through invocations of methods defined in the pattern’s interface
Example:
Let’s take an example of text editor, it let’s you change the state of text that is typed i.e. if you have selected bold, it starts writing in bold, if italic then in italics etc.
First of all we have our transformation functions
const upperCase = inputString => inputString.toUpperCase() const lowerCase = inputString => inputString.toLowerCase() const defaultTransform = inputString => inputString
Then we have our editor
class TextEditor {
constructor(transform) {
this._transform = transform
}
setTransform(transform) {
this._transform = transform
}
type(words) {
console.log(this._transform(words))
}
}
And then it can be used as
const editor = new TextEditor(defaultTransform)
editor.type('First line')
editor.setTransform(upperCase)
editor.type('Second line')
editor.type('Third line')
editor.setTransform(lowerCase)
editor.type('Fourth line')
editor.type('Fifth line')
// Output:
// First line
// SECOND LINE
// THIRD LINE
// fourth line
// fifth line
Template Method Design Pattern
Real world example
Suppose we are getting some house built. The steps for building might look like Prepare the base of house Build the walls Add roof Add other floors The order of these steps could never be changed i.e. you can’t build the roof before building the walls etc but each of the steps could be modified for example walls can be made of wood or polyester or stone.
In plain words
Template method defines the skeleton of how certain algorithm could be performed but defers the implementation of those steps to the children classes.
Wikipedia says
In software engineering, the template method pattern is a behavioral design pattern that defines the program skeleton of an algorithm in an operation, deferring some steps to subclasses. It lets one redefine certain steps of an algorithm without changing the algorithm’s structure.
Example:
Imagine we have a build tool that helps us test, lint, build, generate build reports (i.e. code coverage reports, linting report etc) and deploy our app on the test server.
First of all we have our base class that specifies the skeleton for the build algorithm
class Builder {
// Template method
build() {
this.test()
this.lint()
this.assemble()
this.deploy()
}
}
Then we can have our implementations
class AndroidBuilder extends Builder {
test() {
console.log("Running android tests");
}
lint() {
console.log("Linting the android code");
}
assemble() {
console.log("Assembling the android build");
}
deploy() {
console.log("Deploying android build to server");
}
}
class IosBuilder extends Builder {
test() {
console.log("Running ios tests");
}
lint() {
console.log("Linting the ios code");
}
assemble() {
console.log("Assembling the ios build");
}
deploy() {
console.log("Deploying ios build to server");
}
}
And then it can be used as
const androidBuilder = new AndroidBuilder() androidBuilder.build() // Output: // Running android tests // Linting the android code // Assembling the android build // Deploying android build to server const iosBuilder = new IosBuilder() iosBuilder.build() // Output: // Running ios tests // Linting the ios code // Assembling the ios build // Deploying ios build to server
The JavaScript Design Patterns is a github repository by Pradeep Kumar